1. Field of the Invention
The present invention relates to an apparatus for controlling a driving power of a vehicle equipped with an automatic transmission.
2. Prior Art
In recent years, automatic vehicles, i.e., vehicles equipped with automatic transmissions, have been increasing in number. In automatic vehicles, the gear ratio of the automatic transmission is controlled according to the vehicle speed and throttle valve opening.
In this case, if the accelerator pedal is released when driving downhill, and the vehicle speed nevertheless increases, a gear ratio is automatically shifted up so that an engine braking becomes less effective.
To solve this problem, it is already known to provide an apparatus designed to increase the engine braking effect by shifting a gear ratio down if a vehicle acceleration is not negative, or if a vehicle speed is continuously increased for a predetermined interval, when the accelerator pedal is not depressed.
However, when a gear ratio is shifted down, a problem, that is, a deterioration of drivability arises because an engine speed increases sharply, and an engine braking power becomes excessive.
To address this problem, there has already been proposed a transmission control apparatus designed to increase the engine output by opening the throttle valve when a gear ratio is shifted down while the accelerator pedal is not depressed (refer to Unexamined Japanese Patent Publication No. 5-229368).
This, however, cannot solve the problem of deteriorated driveability due to a torque shock caused by time lags, one from an opening of a throttle valve to increase of the engine speed and the other from an issue of a shift command (an activation of a solenoid) until an actual changeover of a gear ratio.
FIG. 2 is a diagram for explaining the problem, and engine speed Ne, throttle valve opening .theta., engine torque Te, and drive shaft torque Ts are shown in this order from the top to the bottom. Time is plotted along the abscissa.
The downshift command is issued at time t.sub.0, the throttle valve is opened at time t.sub.1, and the engine torque Te begins to increase at time t.sub.3.
In response to the downshift command issued at time t.sub.0, the gear-shift solenoid of the automatic transmission is activated at time t.sub.2.
If a gear-ratio changeover is actually performed at time t.sub.3, that is, at the same time when the engine torque Te begins to increase after the activation of the gear-shift solenoid, the drive shaft torque Ts decreases smoothly as shown by the solid line and no torque shock occurs.
On the other hand, if the gear-ratio changeover is performed at time t.sub.4 after the engine torque Te has increased due to the activation of the gear-shift solenoid, a torque shock occurs because the drive shaft torque Ts temporarily increases as shown by the dashed line.
While a time interval from an opening of the throttle valve to an increase of the engine torque Te can be estimated from the engine operating condition, it is not possible to estimate a time interval between an activation of the gear-shift solenoid and an actual changeover of the gear-ratio since it is influenced by the amount of hydraulic fluid filled in the piston of the relevant gear-shift clutch. It is therefore difficult to control so that the timing of the gear-ratio changeover coincides with the timing of the engine torque increase.
The problem of excessive engine braking also occurs when the vehicle is put in a non-driven condition after downshifting, even if the accelerator pedal is depressed.